Algae Growth on Heat Pumps: When It Becomes a Problem
by Energie Solutions | May 20, 2025 | Air Source Heat Pumps, Heat Pumps, Technical Guides
Air source heat pumps (ASHPs) are a fantastic low-carbon way to heat homes and water, especially in the UK. However, like most things exposed to outdoor elements, they can face occasional environmental challenges, including algae growth.
At first glance, a bit of green buildup around your unit might seem unsightly but harmless. In most cases, it is. Algae tends to develop in damp, shaded areas like the base of the unit or in areas with standing water. But there’s one part of your heat pump where algae should never be allowed to take hold: the evaporator coil.
Why Does Algae Grow on Heat Pumps?
Because ASHPs are installed outdoors, they are exposed to moisture, dirt, and organic debris. These conditions, especially when combined with warm weather and limited sunlight, can create the perfect habitat for algae growth.
Common places algae may appear include:
- Around the condensate drainage area
- At the bottom pan of the outdoor unit
- In and around the casing during periods of high humidity
Most of the time, this is purely cosmetic or a minor maintenance issue. However, it becomes more serious when algae makes it way onto the evaporator coil – a critical component for heat transfer.
When Algae Reaches the Evaporator Coil
The evaporator coil in ASHPs is responsible for drawing heat from the outside air and transferring it to the refrigerant, which then delivers heat into your home. This coil must remain clean and free from obstructions to function effectively.
If algae begins to grow on the evaporator coil, several performance issues can arise:
- Impaired Heat Transfer – Algae creates a slimy layer that insulates the coil surface, reducing its ability to absorb heat from the air. This forces the system to run longer to achieve desired temperatures, increasing energy consumption.
- Reduced System Efficiency – A fouled evaporator coil means the unit must work harder, which lowers its Coefficient of Performance (COP) and increases your heating bills.
- Coil Freezing – When airflow is restricted or heat absorption is hindered, the coil can drop below freezing temperatures. Moisture in the air may freeze on the coil, further obstructing operation and potentially shutting down the system.
- Increased Wear and Tear – Running under strain causes the compressor and fans to cycle more frequently, reducing the lifespan of your unit and increasing the likelihood of breakdowns.
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